1. Field of the Invention
The present invention relates to a device for the storage of data elements, in particular, to the storage of so-called AUDIO-type data elements whose nature is such that, from time to time, they can tolerate storage defects without causing highly perceptible errors during use, notably when they are heard. The device may also be used for the storage of image data elements, notably when they are compressed. Within this disclosure, therefore, the term AUDIO is more generally used to designate audio, video, and other types of data elements. In order to reduce the cost of storage of these AUDIO information elements, it is desired to use AUDIO-type static or dynamic electronic memories, that is, memories that contain defective memory cells.
For such a structure to be possible, the error rate should not exceed a certain maximum value and, furthermore, errors should be presented so that they are evenly distributed during the re-utilization, i.e., during the retransmission, of the AUDIO signal. In a known way, to limit the possible consequences of these defects, an error detection and correction code can be added to the digital information elements representing the AUDIO signal. Error detection and correction codes are fairly efficient for isolated defects, and are of a known type. They necessitate the implementation of error-correction algorithms.
2. Discussion of the Related Art
Two constraints arise when it is sought to use AUDIO-type memories. The first constraint is not to exceed a given error rate (TMAX) in the memory. Second, the defects must be uniformly distributed in a memory map of the memory. AUDIO-type random-access memories (RAMs) include two types of defects. First, they include defective memory cells that are isolated. Second, they include defective memory cells that are grouped together. Defects of the latter type are present notably when there are defective memory rows or columns. It can be seen that the latter type of defect is not compatible with the second constraint referred to above. The choice of AUDIO memories should thus be limited to those whose rate of isolated defective memory cells is lower than TMAX and whose rate of grouped defective memory cells is zero. The drawback of this situation is that it is necessary to sort out the audio memories to find out which memories can be used. Furthermore, certain memories are rejected at the end of the sorting operation
A known method for resolving this problem is to check the state of the memory and of the cells to be addressed before power is turned on and before the data elements are written. When defects are observed, a table of the defects is prepared. This table is saved, and the addresses intended for the defective memory cells are rerouted to redundant memory cells. This method has the drawback firstly of being complicated and, secondly, of prompting a loss of time during use. A result of a use such as this is also that the rate of use of the memory is no more than 100% and that the mean bit rate of the memory is slower. The drawback of these prior art techniques is that the development of an application with memories such as these depends on a so-called "zero defect" quality of the memories used.